Ice, on the Rocks

“Ice, on the Rocks” was first published in the March-April 2005 issue of “Cliff Notes.”

Of all the forces that have done their part to shape the Palisades over the eons — the movement of tectonic plates, the relentless play of wind and rain upon rock, the sculpting hand of the Hudson River — none may have played more of a profound role than ice.

To understand how this force — simple frozen water — has played its role, an overview of how geologists believe the Palisades were formed is in order.

Geologists believe that the formation of rock that would one day become our familiar Palisades Cliffs first emerged from deep within the earth around 200 million years ago. Yet, how does one imagine something like “200 million years”? The span is literally inhuman, by any reckoning. We’ve found over the years that using analogy can be helpful, and the analogy we’ve developed to take people across that vast span goes like this:

Let’s imagine that we could compress that 200 million years into one “day,” one 24-hour time period. (In this analogy, then, one “hour” equals approximately 8.3 million years.) We begin at “midnight.” All of the earth’s continents as we know them are at present combined into a single “super-continent” that geologists call Pangaea. Dinosaurs roam the surface of Pangaea. But Pangaea is breaking apart, and the stress of Pangaea’s disintegration is causing geologic turmoil. Vast quantities of molten rock, or magma, are getting released from miles below the surface to find their way to the top. In some places, magma violently bursts above the surface (to become lava), creating volcanoes. At other locales, the magma follows a different course. Here, where the Palisades will stand at the end of our “day,” the magma gets diverted into horizontal fissures running through the soft sandstones and shales — until “recently,” a seabed — into which it is being pushed upward. Instead of reaching the surface, it flows laterally, like an underground river, more than 40 miles long, 5 or 6 miles wide, up to a thousand feet thick. It creates what is called a sill, buried deep beneath layers of sandstone and shale. It literally bakes the rocks with which it comes into contact, metamorphosing some of them into the red, crumbly rock one still finds beneath the ancient sill (good examples of this metamorphic rock, “baked” shale, can be found behind the Kearney House).

For the next “hour” or so, the sill cools and hardens beneath the ground, the molten rock crystallizing into the hard Palisades rock called diabase.

Still well before “dawn,” further convulsions caused by the breakup of Pangaea result in the entire region being shifted down between 17 and 20 degrees to the west, lifting the eastern edge of the hardened sill (that raised edge will become the Palisades Cliffs).

Throughout the course of the rest of the “day,” as the pieces of Pangaea slowly move into the positions of the continents as we think of them, the forces of erosion — wind and rain, streams and gravity — will relentlessly whittle away at the softer rock above and around the buried sill. Little by little, parts of the buried sill, especially along its easternmost, raised edge, begin to emerge above the surface. (Just as a point of reference, it is believed that at around 4:30 in the “afternoon,” i.e., about 65 million years ago, a meteor or comet slams into what is today the Yucatan Peninsula; the global winter created by this event will spell the doom of the dinosaurs, the ascendancy of mammals.) An ancestral stream flowing roughly along the path of today’s Hudson also does its part to chisel away the softer rock to reveal more of the edge of the sill. By “supper time,” the continents are more or less in their present positions. (Around “11 PM,” meanwhile, the earliest ancestors of human beings show up in Africa…)

Throughout the “evening,” the creatures living here in the future New York–New Jersey metropolitan area enjoy a subtropical climate. Then, a “minute” or two before the end of our “day,” a series of Ice Ages overtake the earth. The causes of these dramatic global shifts in climate are not yet well understood, even if the results are. As temperatures drop in the north, snow fails to melt in places it previously had. Winter after winter, new snow piles upon the old, until that pile grows to hundreds of feet tall. Its own weight compresses the snow at the bottom of the pile, until it begins to behave like something other than snow. It begins to behave like a liquid.

It begins to flow.

Moving in the path of least resistance — generally, south in this hemisphere — the massive pile of frozen snow and ice begins, in essence, to smoosh itself across the ground before it. A glacier has been born. Slowly — its progress typically measured in feet, even inches, per year — this frozen mass of water grinds over the surface of the earth, scraping away anything loose in its path. Vegetation, even mighty forests, loose rocks, soil — all of it gets inexorably pushed in front of the plough of the glacier. Called moraine, this line of debris will be left behind when the glacier finally begins to melt back (or, retreats), a deep pile of rubble, called terminal moraine, which marks the southernmost reach of the glacier’s leading edge. In the case of the last (or, perhaps better put, most recent) glacier, the Wisconsin Ice Sheet, its terminal moraine is found as far south as Newark, New Jersey; much of the high ground in Long Island is likewise made up of terminal moraine from the Wisconsin Ice Sheet. Within the terminal moraine, no doubt, are vast quantities of the sandstones and shales that once encased the Palisades. Also, there are pieces of the sill itself in the terminal moraine, the “talus” that had fallen from the cliffs before the glacier came. The cliffs when the glacier retreats, in other words, are as sheer from top to bottom as they ever will be; new talus will begin to accumulate at their base.

Now it is a few “seconds” before “midnight.”

As the glacier continues to retreat (along its retreat route it will scatter boulders that failed to dislodge at its terminus; these are called erratics), the first human beings arrive on the scene, hunters and gatherers of the forests and wetlands that emerge as the ice slowly returns north.

(Again for point of reference: a Florentine navigator named Verrazano will make the first written record of the Hudson in 1524, almost four centuries ago — and just inside of a quarter of a second before the final “midnight“ of our peculiar “day”…)

Most scientists who study climate believe that we are most likely in what is termed an “interglacial period” — between two Ice Ages. (The next Ice Age, however, remains most likely millennia in the future; and, of course, there are the ongoing concerns about the effects that human activity may have on climate.) Yet even without a glacier bearing down upon us, ice continues to be a powerful shaper of the Palisades.

When the diabase cooled, it naturally formed into vertically arranged columns, the tall pillars of rock that gave the cliffs their name (they reminded someone of a palisade, or stockade-type fence). The cliff face and its columns are rife with cracks. Into these cracks each year rainwater collects. In the winter, that water freezes. Water, unlike most other substances on earth, expands when it freezes. (The same property causes ice cubes in a freezer tray to rise above the rim.) That expansion can exert a force on the order of 2,000 pounds per square inch. This force in turn enlarges fissures in the rock face and can result in rockfalls, though often months later, during the spring thaw.

It all reminds us in a way of the childhood game of Paper, Scissors, Rock — except, in this version, Water, especially in its frozen form, usually wins…

EN

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